Wide angle optical filters

ABSTRACT

A thin-film optical interference filter designed and manufactured to have high transmission of blue light and high reflectance of infrared light. The novel feature of this filter rests in the thin-film layer design at the top and bottom of a quarterwave stack and the manufacturing technique by which this thin-film design is deposited. The thin-film layer design suppresses the so called halfwave holes which occur at wide angles of incidence of the incoming light. This desirable effect is achieved by the proper layer thickness and refractive index control in the manufacturing process.

STATEMENT OF GOVERNMENT INTEREST

The Government of the United States of America has certain rights inthis invention pursuant to Contract No. N00039-84-C0643, SubcontractSD03-01531, with the United States Navy.

p This is a continuation-in-part of Ser. No. 943,274, filed Dec. 17,1986, assigned to the assignee hereof now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to a thin-film optical interferencefilter designed and manufactured to have high-transmittance of bluelight and high reflectance of infrared light.

Optical interference filters of the long-wave pass and short-wave passtypes are normally based upon quarterwave stacks. See, for example, H.A. Macleod, "Thin-film Optical Filters," American Elsevier, N.Y. (1969).

A quarterwave stack generally consists of alternating layers or "series"of high-reflectance materials and low-reflectance materials. See, forexample, A. Thelen, "Equivalent Layers in Multilayer Filters," J. Opt.Soc. Am.. 50:1533-1538 (1966). See also U.S. Pat. No. 2,412,496, issuedDec. 10, 1946, to Dimmick.

One problem typically encountered in the use of optical filters basedupon quarterwave stacks is known as a "halfwave hole." The halfwaveholes appear as dips or minima in the transmittance curve, occurringespecially at incident angles approaching 50°. See, H. A. Macleod,"Performance Limiting Factors in Optical Coatings," Proceedings of theLos Alamos Conference on Optics, '81, D. H. Liebenberg, ed., SPIE,288:580-586 (1981).

The nature of the halfwave holes is such that monitoring errors, whichcause departure from the strict halfwave thickness of the layers,invariably lead to transmittance dips in the transmission band. Nosatisfactory compensation for wide-angle filters has been discovered forthis error, which must be kept as small as possible.

Another cause of a halfwave hole is layer dispersion. A layer whichappears as a quarterwave at 900 nm does not necessarily appear as ahalfwave at 450 nm. This is due to the fact that the film's refractiveindex may increase at shorter wavelengths. Since the variation inrefractive index is generally less for low-index layers, this variationcauses a distinction between the wavelength for which the high-indexlayers are halfwaves from that for which the low-index layers arehalfwaves. Thus, the high-and low-index layers are not halfwaves at thesame wavelength, and a "hole" appears.

One approach to solving the "halfwave hole" problem includes ensuringthat, in spite of dispersion, the layers are halfwaves at precisely thesame wavelengths. This approach strives to eliminate thickness errors asfar as possible. An optical interference filter incorporating thesecorrections was disclosed in a report entitled "SLC Cesium AtomicResonance Filter Interference Coatings," by H. A. Macleod et al., FinalReport for Navy Contract No. N66001-82K-0187 (September 1983).

The present invention provides an alternate solution to the halfwavehole phenomenon.

SUMMARY OF THE INVENTION

The present invention is directed to a thin-film optical interferencefilter which provides a high-transmission of blue light andhigh-reflectance of infrared light.

The unique features of this filter rest in the thin-film layer designand the manufacturing technique by which this film is deposited.

The thin-film layer of the present invention suppresses the halfwavehole problem which occurs at wide angles of incidence of the incominglight. This desirable result is achieved by a combination of the properlayer thicknesses and refractive-index control in the manufacturingprocess.

The novel coating design and fabrication method of the present inventionis described in the detailed description which follows, infra.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 represent the computed reflectance of the thin-filminterference filter of the present invention, in contrast to thecomputed reflectance of a typical quarterwave stack of the prior artshown in FIGS. 3 and 4. In each figure, the percent reflectance is shownversus wavelength, where reflectance is the percentage of light nottransmitted through the filter.

In FIGS. 1 and 2, the reflectance at 0 and 53 degrees angle of incidencerespectively, is illustrated for the filter of the present invention.

In FIGS. 3 and 4, the reflectance at 0 and 53 degrees angle of incidencerespectively, is illustrated for a typical quarterwave stack from theprior art.

FIG. 5 is an enlarged cross-sectional view of a thin-film opticalinterference filter in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A filter in accordance with the invention is designed in part to reflectinfrared light or wavelengths in the range of approximately 850-900 nmat angles of incidence of 0° to approximately 55°. Accordingly, thedesign wavelength, to which the term "quarterwave" pertains, isapproximately 880 nm. The abbreviation "nm" is used herein to denote ananometer which is one billionth of a meter or ten Angstroms.

The quarterwave stack portion of this design, for example, layers 2-24,inclusive, of FIG. 5, is known in the art of interference coatingtechnology. The new features of the stack of the present invention arethe layers at the top and bottom of the stack, e.g., layer 1 in FIG. 5at the top and layers 25-27 in FIG. 5 at the bottom, as well as thenovel method of fabricating the filter.

FIGS. 1 and 2 represent the computed reflectance of a preferredthin-film interference filter of the present invention. FIGS. 3 and 4represent the computed reflectance of a typical quarterwave stack fromthe prior art. FIGS. 1 and 3 are based on the model that the incidentlight is normal to the filter. In FIGS. 2 and 4, the light approachesthe filter at an angle of incidence of 53 degrees from the normal. Asmay be seen from comparisons of FIGS. 1 and 3 and FIGS. 2 and 4,respectively, the percentage reflectance of the invention issubstantially less than that of the prior art for light havingwavelengths ranging between approximately 430 to approximately 480 nmwhich may be characterized herein as blue light.

In a thin-film optical filter in accordance with the invention,approximately 95% or more of blue light passes through the filter,whereas comparable quarterwave optical filters of the prior arttypically transmit a maximum of 85% of blue light. The blue-lighttransmittance advantage of the invention improves with greater angles ofincidence of the incoming light up to approximately 55 degrees angle ofincidence. The ability to transmit a high percentage of blue light atwide angles is advantageous in various applications includingpre-filtering of atomic resonance communications devices. Using this newdesign and the manufacturing process described below to prepare same, acost effective system for manufacturing wide-band optical filters,especially useful in undersea communications devices, has been achieved.

The manufacture of a filter in accordance with the invention may beconducted using a low-refractive index material, preferably silicondioxide, and a high-refractive index material, preferably titaniumdioxide. The layers are deposited, for example, by a known electron beamevaporation technique in a high-vacuum coating machine with planetarysubstrate tooling. Other conventional deposition techniques may beemployed, but electron beam evaporation is the preferred method. Forpurposes herein, an index of refraction will be characterized as being"low" if it is less than or equal to 1.6; as "intermediate," if greaterthan 1.6 and less than 1.9; and as "high," if greater than or equal to1.9, for filters designed to operate in the visible and near-infraredspectra.

During the coating process, an ample supply of oxygen is maintainedwithin the coating machine. This insures that the vaporized titaniummonoxide will combine with oxygen and condense into a titanium dioxidelayer. During deposition, the substrate is maintained at an elevatedtemperature, e.g., at or above 200° C.

It is known that the refractive index of titanium dioxide is affected bythe partial pressure of oxygen, the total pressure in the coatingmachine, and the rate of deposition; so that by careful monitoring ofthese parameters during the deposition process, reproducible control ofthe index of refraction for each layer may be achieved.

Filter size is not a constraint, so long as the physical orientation ofthe evaporation sources to the substrate is maintained and the coatingmachine is well instrumented so that proper coating conditions aremaintained.

The fabrication of the optical coating is performed by applyingsuccessive layers commencing at the substrate at the "bottom" of thestack and ending at the "top" of the stack; see, for example, FIG. 5.The top two layers (preferably both of silicon dioxide), i.e., layers 1and 2 of FIG. 5, are produced by interrupting the deposition process, sothat two layers of the same low-refractive index material are created.This may be accomplished, for example, by stopping deposition, allowingthe substrate to cool, e.g., to about 100° C., and then resumingdeposition. The interruption is preferably conducted for only a shortperiod, for example, for about 5 minutes. This creates an opticalboundary defining the two adjacent layers of silicon dioxide. At thisboundary, optical light interference occurs which minimizes reflectionand maximizes transmission of blue light through the filter.Experimental results show that layer 1 should have a thickness ofapproximately 25% to approximately 67% of the thickness of layer 2 inorder to observe improved transmission of blue light through the filter.Note, layer 2 is the top most layer of the quarterwave stack (see FIG.5).

The gist of the novel method of fabrication of a thin-film opticalfilter is as follows. An optical boundary is formed between two adjacentlayers of the same coating material by applying a first coating on thesubstrate (or on another coating of the filter) at a first temperatureand then applying a second coating of the same material on the firstcoating at a second temperature. In practice, this involves four stepsas follows: (a) heating or cooling the substrate to a first temperature;(b) applying the first coating of material at a first temperature toattain a first coating thickness; (c) heating or cooling the substrateto a second temperature; and (d) applying the second coating of the samematerial at a second temperature to attain a second coating thickness.It is not critical whether the second deposition temperature is greateror less than the first deposition temperature as long as the twodeposition temperatures are different. Because the two adjacent coatingsare applied at different deposition temperatures, the respectivedensities and refractive indices of the two coatings are also different,notwithstanding the fact that the two coatings are of the same material,whereby an "optical boundary" between the coatings has been defined. Inthe preferred embodiment described herein, the second depositiontemperature is cooler than the first temperature so that step (c) isaccomplished simply by interrupting the coating process for a periodsufficient to allow the substrate to cool to the second depositiontemperature. As will be seen, other deposition parameters, such as thedeposition pressure and deposition rate, may be varied between theapplication of the two coatings; however, it is believed that depositiontemperature is the critical control parameter.

In a preferred embodiment of the present invention employing silicondioxide in the top two stack layers, layer 1 has an index of refractionof approximately 1.43 and layer 2 has an index of refraction ofapproximately 1.48. Thus, layer 1 is believed to act in combination withlayer 2 to minimize reflectance of blue light in a way analogous to thatof an ideal decreasing graded index layer.

The three layers at the bottom of the stack, i.e., layers 25-27 of FIG.5, provide an optical interface between the substrate and quarterwavestack which minimizes reflectance of blue light particularly at higherangles of incidence up to approximately 55 degrees. Layers 25-27 of FIG.5 comprise a layer of a low-refractive index material, such as silicondioxide, sandwiched between two substantially thinner layers of ahigh-refractive index material, such as titanium dioxide.Experimentation has demonstrated that this combination of layermaterials and thicknesses at the bottom of the stack maximizes theability of the filter to transmit blue light at high angles ofincidence.

In laboratory examples, the thicknesses of layers 25 and 27 were variedfrom approximately 1/64 to 1/32 of the design wavelength. All examplesdemonstrated beneficial results, i.e., improved transmission of bluelight through the filter. There is no requirement that layers 25 and 27have equal thicknesses. It is believed that a layer or layers thinnerthan 1/64 of the design wavelength would provide beneficial resultsprovided uniform layer thickness can be effectively fabricated.Observations indicate that benefits diminished rapidly when the layerthickness exceeded 1/32 of the design wavelength.

In the laboratory examples investigated, the middle layer, i.e., layer26 of FIG. 5, had a thickness of one-eighth of the design wavelength,plus or minus ten percent of one-eighth of the design wavelength. In allcases, the benefits of the invention were observed as reported.

Laboratory examples further demonstrate that the benefits of theinvention are enhanced when the invention includes the combination ofthe top-most layer of the stack, the three layers at the bottom of thestack, and the intermediate quarterwave stack; this combination is thepreferred embodiment described herein. However, substantial benefits maybe obtained by employing the three layers at the bottom of the stack andthe quarterwave stack without the layer above the quarterwave stack.

The present invention will be further illustrated with reference to thefollowing example which aids in its understanding, but which is not tobe construed as a limitation thereof.

WORKING EXAMPLE

Manufacture of the preferred filter of the present invention wasconducted using silicon dioxide as the low-refractive index material,N_(L) =1.48, and titanium dioxide as the high-refractive index material,N_(H) =1.9 to 2.3, where N is the index of refraction.

The layers were deposited by electron beam evaporation in a high-vacuumcoating machine with planetary tooling. During evaporation, oxygen wasintroduced into the system so that titanium dioxide was formed byoxidation of vaporized titanium monoxide.

The substrate temperature was maintained at about 200° C. throughout thedeposition of the quarterwave stack. The substrate material wasborosilicate glass.

The top two stack layers of silicon dioxide were produced byinterrupting the deposition, so that two layers of the same materialwith an optical boundary therebetween were created. This wasaccomplished by stopping the deposition process, allowing the system tocool from 200° C. to about 100° C., and then resuming deposition. Theinterruption was conducted for about 5 minutes. This created the desiredoptical boundary. The refractive index of layer 2 of silicon dioxide wasapproximately 1.48, and the refractive index of the layer 1 of silicondioxide was approximately 1.43.

The layers of the preferred optical filter of the present invention andtheir respective coating parameters are set forth in Table I below. Inthis Table, "N" is the index of refraction. "D" is coating thickness. "D(monitor)" is the coating thickness on an Inficon IC-6000 thicknessmonitor. "D (filter)" is the actual coating thickness on theborosilicate substrate. "D (optical)" is the design thickness, measuredin fractions of the design wavelength. "Rate" is the rate of depositionmeasured by the Inficon monitor in nanometers per second. "Oxygenpressure" is the partial pressure of oxygen admitted to the coatingmachine in units of 10⁻⁴ Torr. "Total pressure" is the total pressurewithin the coating machine in units of 10⁻⁴ Torr. SiO₂ is anabbreviation for silicon dioxide, and TiO₂ is an abbreviation fortitanium dioxide.

In the Table, values for D (monitor), Rate, Oxygen Pressure, and TotalPressure are observed values. Values for N, D (optical), and D (filter)were obtained from a computer simulation. These values are believed tobe accurate because the observed filter performance was consistent withthe performance predicted by the computer simulation. The lower indicesof refraction in layers 25 and 27 are the result of the lower depositionrates required to attain the extreme thinness of these layers.

Another high-index material that may be employed in place of titaniumdioxide is zinc sulfide. Other possible low-index materials aremagnesium fluoride, cryolite (but not with titanium dioxide), andthorium fluoride. These suggested alternate materials are intended asexamples and by no means comprise an exhaustive list.

FIGS. 1 and 3 show the performance of this embodiment of the invention.As commented on above, the filter transmits a substantially higherpercentage of blue light than its prior art counterparts while havinghigh reflectance of infrared wavelengths.

The present invention has been described in detail, including thepreferred embodiments thereof. However, it will be appreciated thatthose skilled in the art, upon consideration of the present disclosure,may make modifications and/or improvements on this invention and stillbe within the scope and spirit of this invention as set forth in thefollowing claims.

                                      TABLE I                                     __________________________________________________________________________    EXAMPLE COATING PARAMETERS                                                                                        Oxygen                                                                              Total                                           D (monitor)                                                                          D (filter)                                                                         D (optical)                                                                          Rate Pressure                                                                            Pressure                            Layer                                                                             Material                                                                           N  (nm)   (nm) (wavelength)                                                                         (nm/sec)                                                                           (10.sup.-4 Torr)                                                                    (10.sup.-4 Torr)                    __________________________________________________________________________    (Top of Stack)                                                                1   SiO.sub.2                                                                          1.43                                                                             109.2  82.5 0.125  1.5  1.6   2.6                                 2   SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 3   TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 4   SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 5   TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 6   SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 7   TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 8   SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 9   TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 10  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 11  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 12  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 13  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 14  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 15  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 16  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 17  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 18  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 19  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 20  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 21  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 22  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 23  TiO.sub.2                                                                          2.25                                                                             136.1  108.9                                                                              0.25   1.2  2.1   2.6                                 24  SiO.sub.2                                                                          1.48                                                                             219.1  165.5                                                                              0.25   1.5  1.6   2.6                                 25  TiO.sub.2                                                                          2.10                                                                             7.9    6.3  0.015  0.2  2.1   2.6                                 26  SiO.sub.2                                                                          1.48                                                                             109.8  82.9 0.125  1.5  1.6   2.6                                 27  TiO.sub.2                                                                          2.10                                                                             7.9    6.3  0.015  0.2  2.1   2.6                                 (Bottom of Stack)                                                             __________________________________________________________________________

What is claimed is:
 1. A thin-film optical interference filter havinghigh-transmission of blue light and high-reflectance of infrared light,said filter having a predetermined design wavelength, said filtercomprising:(a) a glass substrate; (b) a first or bottom coating of ahigh-refractive index material applied on said substrate, said firstcoating having a first thickness of less than one-sixteenth of saiddesign wavelength; (c) a second coating of a low-refractive indexmaterial applied on said first coating, said second coating having asecond thickness greater than one-sixteenth and less thanthree-sixteenths of said design wavelength; (d) a third coating of ahigh-refractive index material applied on said second coating, saidthird coating having a third thickness of less than one-sixteenth ofsaid design wavelength; and (e) a quarterwave stack applied on saidthird coating, said quarterwave stack including a plurality of layers ofalternating high- and low-refractive index materials, said quarterwavestack having a design wavelength being matched approximately to saidpredetermined design wavelength of said filter.
 2. An optical filter asdescribed in claim 1 wherein said filter further includes a fourthcoating of low-refractive index material applied on said quarterwavestack, said fourth coating having a thickness of less than one-quarterof said design wavelength.
 3. An optical filter as described in claim 1wherein said first and third coatings are formed from titanium dioxideand said second coating is formed from silicon dioxide.
 4. An opticalfilter as described in claim 2 wherein said fourth coating is formedfrom silicon dioxide.
 5. An optical filter as described in claim 1wherein the thicknesses of said first and third coatings are each lessthan one thirty-second of said design wavelength and the thickness ofsaid second coating is one-eighth of said design wavelength plus orminus one-tenth of one-eighth of said wavelength.
 6. An optical filteras described in claim 2 wherein the thickness of said fourth coating isbetween approximately twenty-five percent to approximately sixty-sevenpercent of one-quarter of said design wavelength.
 7. An optical filteras described in claim 1 wherein said design wavelength is approximately880 nanometers.